2004A&A...421.1087H

We have studied the ratio of ¹³CO and C¹⁸O column densities to the extinction A_J of background field stars in the direction of three globules: B 133, B 335, L 466. Selected positions in the globules were observed in the ¹²CO(J=1-0) line and in J=1-0 and J=2-1 transitions of ¹³CO and C¹⁸O using the 15-m Swedish-ESO Submillimetre Telescope (SEST) in Chile. One of the clouds, L 466, was also mapped in the C¹⁸O(J=1-0) line. In addition to the N(¹³CO)/A_J and N(C¹⁸O)/A_J ratios also the ratios of N(¹³CO) and N(C¹⁸O) to A_V and N(H₂) are given. These ratios were found to vary from cloud to cloud so that they are larger in B 335 than in B 133 and L 466. These variations are thought to primarily arise from variations of the column density ratio of CO and its isotopes to H₂. The alternative explanation in terms of the N(H₂)/A_J ratio being larger in starforming regions (B 335) than in more quiescent regions lacking star formation (B 133 and L 466) appears less likely. We also discuss the evidence for N(CO)/A_J variations on the basis of previous observations in star forming and non-star forming dark clouds.